Power Device

ABSTRACT

The present invention provides a power device including a converter part ( 2 ) for converting an AC voltage to a DC voltage; an inverter part ( 3 ) for converting the DC voltage outputted from the converter part ( 2 ) to the AC voltage; and a transformer ( 4 ) having an inductance forming a series resonance circuit together with an electrostatic capacity of a load ( 20 ) to boost the AC voltage outputted from the inverter part ( 3 ). In the power device, an inductance ( 7 ) is connected to the output part of the inverter part ( 3 ) in parallel with the transformer ( 4 ). Thus, in a discharge part  6  provided in the load ( 20 ), since when a discharge is not generated, a recovery current is not supplied to a circulating current diode in the inverter part ( 3 ), or the quantity of the recovery current is reduced, the heat generation of the circulating current diode can be suppressed without increasing the number of elements of the circulating current diode.

TECHNICAL FIELD

The present invention relates to a technique of a power device thatsupplies AC voltage to a load having a discharge part to generate adischarge.

BACKGROUND ART

FIG. 7 shows the configuration of a usual power device for a gas laseroscillator disclosed in JP-A-9-129953. The AC voltage of a commercialpower source 1 is converted to a DC voltage in a converter part 2 andinputted to an inverter part 3. In the inverter part 3, switchingelements are turned on and off by a gate signal of a gate signal outputcircuit 10 and the DC voltage is converted to a square wave AC voltage.The output voltage of the inverter part 3 is boosted by a high frequencytransformer 4 having an inductance L and applied to a part betweendielectric electrodes 5 a and 5 b having an electrostatic capacity C, sothat a discharge 6 is generated. A discharge current supplied betweenthe dielectric electrodes 5 a and 5 b has its quantity set by a commandvalue outputted from an NC device 9. The gate signal output circuit 10outputs the gate signal to the inverter part 3 under a dischargefrequency fso(>½π√ (LC)) by a PWM control on the basis of the outputvalue of a discharge current detecting circuit 8 for detecting thedischarge current and the command value of the NC device 9.

The operation of the inverter part 3 carried out when the discharge 6 isgenerated (refer it to as during discharge on period, hereinafter) isdescribed below. FIG. 8 shows the configuration of the inverter part 3in the related art. The inverter part 3 includes switching elements 11a, 11 b, 11 c and 11 d and circulating current diodes 12 a, 12 b, 12 cand 12 d connected in parallel with them. FIG. 9 shows examples of thewave forms of an output voltage and a current of the inverter part 3when the inverter part 3 is controlled in accordance with the PWMsystem. The plus of the wave form of the voltage and the wave form ofthe current in FIG. 9 indicate that in the wave form of the voltage, anarrow mark of the output voltage is directed toward a plus (a highpotential) side in FIG. 8 and, in the wave form of the current, theoutput current flows in the direction of an arrow mark in FIG. 8. Atthis time, to highly efficiently reduce the switching loss of theelements forming the inverter part 3, the inverter part 3 ordinarilyoperates under the frequency fso(>½π√ (LC)) slightly higher than aseries resonance frequency fr (=½π√ (LC)) determined by the inductance Lof the high frequency transformer 4 and the electrostatic capacity C ofthe dielectric electrodes 5 a and 5 b. As shown in FIG. 9, the outputcurrent of the inverter part 3 has a phase lag relative to the outputvoltage of the inverter part. When the switching element 11 b is turnedoff from a state that the switching elements 11 a and 11 b are turned on(during t1 shown in FIG. 9), a circulating current If shown by a brokenline in FIG. 8 is supplied to the circulating current diode 12 c in thedirection of the plus (during t2 shown in FIG. 9). Under this state, theswitching element 11 c is turned on, and then, the switching element 11d is turned on. When the switching element 11 d is turned on, that is,at a point B in FIG. 9, a backward voltage is applied to the circulatingcurrent diode 12 a, however, in the point B, since the circulatingcurrent If flows in the direction of the plus, a forward current is notsupplied to the circulating current diode 12 a. Accordingly, in thecirculating current diode 12 a, a recovery current is not generated.

Similarly, when the switching element 11 d is turned off from a statethat the switching elements 11 c and 11 d are turned on (during t3 shownin FIG. 9), a circulating current If′ is supplied to the circulatingcurrent diode 12 a in the direction opposite to that of the circulatingcurrent If shown by the broken line in FIG. 8, that is, in the directionof the minus (during t4 as shown in FIG. 9). Under this state, theswitching element 11 a is turned on, and then, the switching element 11b is turned on. When the switching element 11 b is turned on, that is,at a point A shown in FIG. 9, a backward voltage is applied to thecirculating current diode 12 c, however, at the point A, the circulatingcurrent If′ flows in the minus direction, a forward current is notsupplied to the circulating current diode 12 c. Accordingly, a recoverycurrent is not generated in the circulating current diode 12 c.

As described above, in the related art, the output current ordinarilyhas the phase lag relative to the output voltage, so that when thedischarge is generated, the recovery current is adapted not to begenerated in the circulating current diode.

However, when the discharge 6 is not generated (refer it to as duringdischarge off period, hereinafter), the discharge current is notcontinuously supplied until the discharge is generated by applying thevoltage to the dielectric electrodes, though the voltage is applied. Thewave forms of the output voltage and the current of the inverter part 3in this case are, for instance, shown in FIG. 10. The plus of thevoltage wave form and the current wave form shown in FIG. 10 indicatesthat in the wave form of the voltage, an arrow mark of the outputvoltage is directed toward a plus (a high potential) side in FIG. 8, andin the wave form of the current, the output current flows in thedirection of an arrow mark in FIG. 8. As shown in FIG. 10, the outputcurrent of the inverter part 3 during discharge off period has a waveform asynchronous with the wave form of the output voltage. Thisphenomenon arises because of a reason why when the discharge isgenerated between the dielectric electrodes 5 a and 5 b, a gap betweenthe dielectric electrodes 5 a and 5 b serves as a DC resistancecomponent, however, when the discharge is not generated, the gap servesas a capacitance, the circuit is equivalent to a circuit in which thecapacitance is inserted in series to the dielectric electrodes 5 a and 5b. Thus, since the impedance and the resonance frequency of the circuitchange, a dark current having a peak or a frequency determined by themflows. As described above, since the circuit is equivalent to thecircuit in which the capacitance is inserted in series to the dielectricelectrodes 5 a and 5 b, the electrostatic capacity of the entire part ofthe discharge part is ordinarily decreased and the resonance frequencyrises.

As shown in FIG. 10, when the dark current flows, at a point A when theswitching element 11 b in FIG. 10 is turned on, the switching element 11b is turned on and the backward voltage is applied to the circulatingcurrent diode 12 c. However, at the point A, the dark current issupplied in the direction of the plus, namely, from the switchingelement 11 a to the high frequency transformer and to the circulatingcurrent diode 12 c. Since the forward current is supplied to thecirculating current diode 12 c, the recovery current is supplied to thecirculating current diode 12 c. Accordingly, an abnormal heat generationarises in the circulating current diode 12 c.

Similarly, at a point B when the switching element 11 d in FIG. 10 isturned on, the switching element 11 d is turned on and the backwardvoltage is applied to the circulating current diode 12 a. However, atthe point B, the dark current is supplied in the direction of the minus,that is, from the switching element 11 c to the high frequencytransformer and to the circulating current diode 12 a. Since the forwardcurrent is supplied to the circulating current diode 12 a, the recoverycurrent is supplied to the circulating current diode 12 a. Thus, anabnormal heat generation arises in the circulating current diode 12 a.

As described above, in the power device for the gas laser oscillatorhaving a load discontinuous between during discharge on period andduring discharge off period, to meet the above-described problemgenerated during discharge off period according to the related art, thenumber of the circulating current diodes connected in parallel can bemerely increased to distribute the loss of the circulating current diodedue to the recovery current. Meanwhile, in recent years, a dischargefrequency in the power device for the gas laser oscillator becomesprogressively high and the response speed of an employed circulatingcurrent diode is requested to be high. Thus, the loss due to therecovery current is also increased. Accordingly, many relativelyexpensive and high speed diodes need to be used in parallel, whichresults in a very serious problem in view of the rise of a cost and theincrease of a mounting space.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to prevent a circulatingcurrent diode of an inverter part from entering a recovery mode orreduce the entering operation of the circulating current diode to therecovery mode in all operating state in a power device having a loaddiscontinuous between during discharge on period and during dischargeoff period so that the heat generation of the circulating current diodeby the recovery current can be reduced and to provide a more compact andinexpensive power device.

A power device according to the present invention includes a converterpart for converting a commercial AC voltage to a DC voltage by arectifying element; an inverter part for converting the DC voltageoutputted from the converter part to an AC voltage of high frequency;and a high frequency transformer having an inductance forming a seriesresonance circuit with an electrostatic capacity of dielectricelectrodes of a load to boost the high frequency AC voltage outputtedfrom the inverter part to a high voltage. In the power device, aninductance is connected to the output part of the inverter part inparallel with the high frequency transformer.

The power device according to the present invention includes a converterpart for converting a commercial AC voltage to a DC voltage by arectifying element; an inverter part for converting the DC voltageoutputted from the converter part to an AC voltage of high frequency;and a high frequency transformer having an inductance forming a seriesresonance circuit with an electrostatic capacity of dielectricelectrodes of a load to boost the high frequency AC voltage outputtedfrom the inverter part to a high voltage, and an inductance is connectedto the output part of the inverter part in parallel with the highfrequency transformer. Accordingly, since when a discharge in the loadis not generated, a recovery current is not supplied to a circulatingcurrent diode in the inverter part, or the quantity of the recoverycurrent is reduced, the heat generation of the diode can be suppressedwithout increasing the number of elements of the circulating currentdiode, and the miniaturization and the low cost of the power device canbe effectively realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic block diagram of a power device according to a firstembodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of a load side of aninverter part of the power device according to the first embodiment ofthe present invention.

FIG. 3 shows wave forms of an output voltage and current duringdischarge on period of the load of the inverter part in the power deviceaccording to the first embodiment of the present invention.

FIG. 4 shows waveforms of an output voltage and current during dischargeoff period of the load of the inverter part in the power deviceaccording to the first embodiment of the present invention.

FIG. 5 is a basic block diagram of a power device according to a secondembodiment of the present invention.

FIG. 6 is a basic block diagram of a power device according to a thirdembodiment of the present invention.

FIG. 7 is a basic block diagram of a usual power device for a gas laseroscillator.

FIG. 8 is a basic block diagram of an inverter part of the usual powerdevice for the gas laser oscillator.

FIG. 9 shows waveforms of an output voltage and current during dischargeon period of the inverter part in the usual power device for the gaslaser oscillator.

FIG. 10 shows waveforms of an output voltage and current duringdischarge off period of the inverter part in the usual power device forthe gas laser oscillator.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 is block diagram showing a power device according to a firstembodiment for carrying out the present invention. The power device isconnected to a gas laser oscillator having dielectric electrodes as aload to generate a discharge between the dielectric electrodes.

In FIG. 1, an AC voltage in a commercial power source 1 is converted toa DC voltage in a converter part 2 and the DC voltage is inputted to aninverter part 3. In the inverter part 3 including a switching elementand a circulating current diode connected in parallel with the switchingelement, the switching element is turned on and off in accordance with agate a signal of a gate signal output circuit 10 and the DC voltage isconverted to a square wave AC voltage. An output voltage of the inverterpart 3 is boosted by a high frequency transformer 4 having an inductanceL and outputted from the power device. An electric power of the highfrequency and high voltage outputted from the power device is appliedto, for instance, a part between dielectric electrodes 5 a and 5 bhaving an electrostatic capacity C of the gas laser oscillator connectedto the power device as the load. Thus, a discharge 6 is generatedbetween the dielectric electrodes 5 a and 5 b to oscillate a laser. Theinductance L of the high frequency transformer 4 and the electrostaticcapacity C of the dielectric electrodes 5 a and 5 b of the laseroscillator as the load form a series resonance circuit. To the outputside of the inverter part 3, a parallel inductance 7 is connected inparallel with the high frequency transformer 4. Further, a dischargecurrent supplied between the dielectric electrodes 5 a and 5 b has itsquantity set by a command value outputted from an NC device 9. The gatesignal output circuit 10 outputs the gate signal to the inverter part 3from an output value of a discharge current detecting circuit 8 fordetecting the discharge current and the command value of the NC device 9under a discharge frequency fso(>½π√)LC)) by a PWM control. Since theinverter part 3 shown in FIG. 1 has the same configuration as that ofthe circuit shown in FIG. 8, this embodiment is described by using thereference numerals shown in FIG. 8.

Now, an operation of this embodiment will be described below. FIG. 2shows an equivalent circuit of a load side of the inverter part 3 in thepower device according to the first embodiment. Further, FIG. 3 showswaveforms of an output voltage and current of the inverter part 3 duringdischarge on period. The plus marks of the voltage wave forms and thecurrent wave forms in FIG. 3 mean that the direction of the arrow markof the output voltage in FIG. 8 is directed to a plus (a high potential)side and the output current flows in the direction the arrow mark. Abroken line i1 in FIG. 3 shows the wave form of a current supplied tothe high frequency transformer 4 side as shown in FIG. 2. This wave formis equivalent to the wave form of the output current of the usual powerdevice without parallel inductance 7 shown in FIG. 9. A dashed line i2in FIG. 3 shows a wave form of a current supplied to the parallelinductance 7 side as shown in FIG. 2. Since the load is a simpleinductive load, the output current has a phase lag relative to theoutput voltage as shown in FIG. 3. A full line i0 shown in FIG. 3 showsthe wave form of the output current of the inverter part 3 as shown inFIG. 2 and is expressed by i0=i1+i2. Therefore, the output current i0shows the wave form increased more by the current i2 supplied to theparallel inductance 7 than the wave forms (FIG. 9) of the output voltageand current of the inverter part 3 during discharge on period in therelated art. As shown in FIG. 3, at a point A when a switching element11 b is turned on, a circulating current flows in the direction of aminus. At a point B when a switching element 11 d is turned on, a statethat the circulating current flows in the plus direction is maintained.Thus, the operation of the inverter part 3 is the same as the usualoperation during discharge on period. Accordingly, a recovery current isnot generated in circulating current diodes 12 a and 12 b in theinverter part 3.

FIG. 4 shows wave forms of an output voltage and current of the inverterpart 3 during discharge off period in the power device according to thisembodiment. The plus marks of the voltage wave forms and the currentwave forms in FIG. 4 mean that the direction of the arrow mark of theoutput voltage in FIG. 8 is directed to a plus (a high potential) sideand the output current flows in the direction the arrow mark. In FIG. 4,a broken line shows a current i1 a supplied to the high frequencytransformer 4 side. This wave form is equivalent to the wave form of thedark current of the usual power device without parallel inductance 7shown in FIG. 10. A dashed line shows a current i2 a supplied to theparallel inductance 7 side. Since the load is a simple inductive load,the output current has a phase lag relative to the output voltage asshown in FIG. 3. A full line shows the output current i0 a of theinverter part 3 and i0 a=i1 a+i2 a is established similarly to thatduring discharge on period.

In FIG. 4, at a point B when the switching element 11 d is turned on,the current i1 a supplied to the high frequency transformer 4 flows inthe direction of the minus (from a switching element 11 c to the highfrequency transformer 4 and to the circulating current diode 12 a).However, the current i2 a supplied to the parallel inductance 7 sideflows in the direction of the plus (from a switching element 11 a to theparallel inductance 7 and to a circulating current diode 12 c). When theabsolute value of i2 a is not lower than the absolute value of i1 a, theoutput current i0 a of the inverter part 3 as the total current of themflows in the direction of the plus (from the switching element 11 a tothe parallel inductance 7 and to the circulating current diode 12 c) orthe current does not flow. Thus, the recovery current is not generatedin the circulating current diode 12 a. When the absolute value of i2 ais smaller than the absolute value of i1 a, the output current i0 aflows in the direction of the minus so that the recovery current isgenerated in the circulating current diode 12 a. However, when aparallel reactor 7 is connected, the quantity of the current supplied inthe direction of the minus can be decreased. Accordingly, the quantityof the recovery current supplied to the circulating current diode 12 acan be effectively reduced more than that obtained when the parallelinductance 7 is not provided.

Similarly, at a point A when the switching element 11 b is turned on,the current i1 a supplied to the high frequency transformer 4 flows inthe direction of the plus (from the switching element 11 a to the highfrequency transformer 4 and to the circulating current diode 12 c).However, the current i2 a supplied to the parallel inductance 7 sideflows in the direction of the minus (from the switching element 11 c tothe parallel inductance 7 and to the circulating current diode 12 a).When the absolute value of i2 a is not lower than the absolute value ofi1 a, the output current i0 a of the inverter part 3 as the totalcurrent of them flows in the direction of the minus (from the switchingelement 11 c to the parallel inductance 7 and to the circulating currentdiode 12 a) or the current does not flow. Thus, the recovery current isnot generated in the circulating current diode 12 c. When the absolutevalue of i2 a is smaller than the absolute value of i1 a, the outputcurrent i0 a flows in the direction of the plus so that the recoverycurrent is generated in the circulating current diode 12 c. However, thequantity of the recovery current supplied to the circulating currentdiode 12 c can be effectively reduced more than that obtained when theparallel inductance 7 is not provided, as in the point B when theswitching element 11 d is turned on. Here, as for i0 a in FIG. 4, thecurrent wave form is shown when the absolute values of i2 a are largerthan the absolute values of i1 a at the points A and B.

In the configuration according to the first embodiment, since therecovery current is not generated in the circulating current diodes inthe inverter 3 both during discharge on period and during discharge offperiod in the load, or the recovery current can be reduced, the heat ofthe circulating current diodes is hardly generated and the power devicecan be miniaturized and a cost can be lowered. Especially, at the pointsA and B in FIG. 4 where a backward voltage is applied to the circulatingcurrent diodes 12 a and 12 c, when the direction of the current i1 asupplied to the high frequency transformer 4 is the same as the forwarddirection of the circulating current diodes 12 a and 12 c, the value ofthe parallel inductance 7 is set so that the absolute value of thecurrent i2 a supplied to the parallel inductance 7 is larger than theabsolute value of the dark current i1 a supplied to the high frequencytransformer 4. Thus, the configuration in which the recovery current isnot generated can be obtained.

The first embodiment is described above by using, as an example, the gaslaser oscillator having the dielectric electrodes to generate thedischarge between the dielectric electrodes as the load of the powerdevice according to the present invention. However, the load is notespecially limited to the gas laser oscillator and any load may be usedthat has an electrostatic capacity and forms a series resonance circuittogether with the inductance L of the high frequency transformer 4.

Second Embodiment

FIG. 5 shows one example of a block diagram for illustrating a secondembodiment for carrying out the present invention. Since the basicconfiguration is the same as that of the first embodiment, the sameparts are designated by the same reference numerals and an explanationthereof is omitted and different points from those of FIG. 1 will bedescribed below.

In FIG. 5, in one end of a parallel inductance 7, a switching device 13is provided that separates the parallel inductance 7 from a circuitduring discharge on period and connects the parallel inductance 7 to thecircuit during discharge off period by a switching signal outputted froman NC device 9 depending on whether or not the discharge 6 is generated.As for the presence or absence of the discharge 6, a discharge on signaloutput circuit 14 decides whether a current value detected by adischarge current detecting circuit 8 is large or small. The dischargeon signal output circuit 14 outputs a signal for discriminating whetheror not the discharge is generated to the NC device 9. In this case, theswitching signal to be outputted to the switching device 13 does notnecessarily need to be outputted from the NC device 9. The switchingsignal may be directly outputted from the discharge on signal outputcircuit 14 and the above-described method does not need to berestrictedly employed. Since an inverter part 3 shown in this embodimenthas the same configuration as the circuit shown in FIG. 8, in thisembodiment, an explanation will be given by using the reference numeralsdesignated in FIG. 8.

Now, an operation of the second embodiment will be described below.Since a discharge current i1D supplied to a secondary side of a highfrequency transformer 4 during discharge on period that is detected bythe current detecting circuit 8 is larger than a dark current i1Esupplied to the secondary side of the high frequency transformer duringdischarge off period (i1D>i1E), a current value “is” represented byi1D>“is”>i1E is set and stored in a storing part of the discharge onsignal output device 14.

A current ix detected by the current detecting circuit 8 is comparedwith the setting value “is” in a comparing part of the discharge onsignal output device 14. When ix is larger than “is”, that is, ix>“is”,the discharge on signal output circuit 14 decides that the discharge isgenerated to transmit a signal to the NC device 9 from the output partof the discharge on signal output circuit 14 so as to separate theparallel inductance 7 from the circuit. The NC device 9 receiving aseparating signal outputs a signal to the switching device 13 todisconnect the circuit. The switching device 13 disconnects the circuitto separate the parallel inductance 7 from the circuit. As a result, theconfiguration of the circuit connected to the output of the inverterpart 3 is the same as the usual configuration (FIG. 7). Accordingly, allthe output current of the inverter part 3 is supplied to the highfrequency transformer 4, so that the wave forms of the output voltageand current shown in FIG. 9 are obtained.

When ix is smaller than “is”, that is, ix<“is”, the discharge on signaloutput device 14 decides that the discharge is not generated andtransmits a signal to the NC device 9 from the output part of thedischarge on signal output device 14 to connect the parallel inductance7 to the circuit. The NC device 9 receiving a separating signal outputsa signal to the switching device 13 to connect the circuit. Theswitching device 13 connects the circuit so as to connect the parallelinductance 7 to the circuit. As a result, the configuration of thecircuit connected to the output of the inverter part 3 is the same asthat of the first embodiment (FIG. 1). Accordingly, the wave forms ofthe output voltage and current of the inverter part 3 show the waveforms shown in FIG. 4. Thus, a recovery current is not generated in acirculating current diode in the inverter part 3 or the recovery currentcan be reduced.

In the configuration according to the second embodiment, the sameeffects as those of the first embodiment can be obtained that since therecovery current is not generated in the circulating current diode inthe inverter part 3 both during discharge on period and during dischargeoff period in the load, or the recovery current can be reduced, the heatof the circulating diode is hardly generated, a power device can beminiaturized and a cost can be lowered. Especially, it is to beunderstood that at the points A and B in FIG. 4 where a backward voltageis applied to the circulating current diodes 12 a and 12 c, when thedirection of the current i1 a supplied to the high frequency transformer4 is the same as the forward direction of the circulating current diodes12 a and 12 c, the value of the parallel inductance 7 is set so that theabsolute value of the current i2 a supplied to the parallel inductance 7is larger than the absolute value of the dark current i1 a supplied tothe high frequency transformer 4, and accordingly, the configuration inwhich the recovery current is not generated can be obtained.

Further, in the first embodiment, during discharge on period, the outputcurrent of the inverter part 3 is increased more by the current suppliedto the parallel inductance 7 than that in the related art and the loadto the inverter part 3 is increased. However, in the second embodiment,since, during discharge on period, the parallel inductance 7 isseparated from the circuit, the current supplied to the parallelinductance 7 is not required. Thus, the value of the output current ofthe inverter part 3 can be made to be equal to that of the related art.Consequently, the efficiency of the power device can be improved and theload of the switching elements 11 a to 11 d of the inverter part 3 canbe reduced.

Third Embodiment

In the first embodiment and the second embodiment, the inductance of thehigh frequency transformer of the power device according to the presentinvention and the electrostatic capacity of the load form the seriesresonance circuit. However, as shown in FIG. 6, even when a load has aninductance and forms an LC series resonance circuit in the load, and apower device does not include a high frequency transformer, a parallelinductance 7 is provided in parallel with the load in the power device,so that an equivalent circuit in the load side of the inverter part ofthe power device is the same as that of FIG. 2 and the same effects asthose of the first embodiment can be obtained. It is to be understoodthat the switching device 13 employed in the second embodiment isprovided so that the same effects as those of the second embodiment canbe obtained.

INDUSTRIAL APPLICABILITY

As described above, the power device according to the present inventionis especially suitably used for supplying an electric power to the gaslaser oscillator that has the dielectric electrodes to generate adischarge between the dielectric electrodes.

1. A power device for supplying an electric power to a load in which anelectrostatic capacity discontinuously changes the power devicecomprising: a converter part for converting an AC voltage to a DCvoltage; an inverter part that has a switching element and a circulatingcurrent diode connected in parallel with the switching element andconverts the DC voltage outputted from the converter part to an ACvoltage; a transformer that has a first inductance forming a seriesresonance circuit with the electrostatic capacity of a load and convertsthe AC voltage outputted from the inverter part; and a second inductanceconnected to an output part of the inverter part in parallel with thetransformer.
 2. The power device according to claim 1, wherein the valueof the second inductance is set so that when the switching element isturned on to apply a backward voltage to the circulating current diode,and a direction of a current supplied to the transformer is the same asa forward direction of the circulating current diode, an absolute valueof the current supplied to the second inductance is to be larger than anabsolute value of the current supplied to the transformer.
 3. The powerdevice according to claim 1, further comprising: a switching unitprovided between one end of the second inductance and the output part ofthe inverter part to switch disconnection and connection of the secondinductance to the output part of the inverter part; and a controller forcontrolling the switching unit.
 4. The power device according to claim3, wherein when a discharge is generated in a discharge part provided inthe load, the controller controls the switching unit to connect thesecond inductance to the output part of the inverter part, and whereinwhen the discharge is not generated, the controller controls theswitching unit to disconnect the second inductance from the output partof the inverter part.
 5. The power device according to claim 4, furthercomprising a current detecting unit for detecting a value of the currentsupplied to the load, wherein the controller compares the value of thecurrent detected by the current detecting unit with a setting value todecide whether or not the discharge is generated in the load.
 6. A powerdevice for supplying an electric power to a load including an LC seriesresonance circuit, in which an electrostatic capacity discontinuouslychanges, the power device comprising: a converter part for converting anAC voltage to a DC voltage; an inverter part that has a switchingelement and a circulating current diode connected in parallel with theswitching element and converts the DC voltage outputted from theconverter part to the AC voltage; and an inductance connected to theoutput part of the inverter part in parallel with the load.
 7. The powerdevice according to claim 6, wherein the value of the inductance is setso that when the switching element is turned on to apply a backwardvoltage to the circulating current diode and a direction of a currentsupplied to the load is the same as a forward direction of thecirculating current diode, an absolute value of the current supplied tothe second inductance is to be larger than an absolute value of thecurrent supplied to the load.
 8. The power device according to claim 6,further comprising: a switching unit provided between one end of theinductance and the output part of the inverter part to switchdisconnection and connection of the inductance to the output part of theinverter part; and a controller for controlling the switching unit. 9.The power device according to claim 8, wherein when a discharge isgenerated in a discharge part provided in the load, the controllercontrols the switching unit to connect the inductance to the output partof the inverter part, and wherein when the discharge is not generated,the controller controls the switching unit to disconnect the inductancefrom the output part of the inverter part.
 10. The power deviceaccording to claim 9, further comprising a current detecting unit fordetecting a value of the current supplied to the load, wherein thecontroller compares the value of the current detected by the currentdetecting unit with a setting value to decide whether or not thedischarge is generated in the load.